1. Field of the Invention
The present invention relates to a zoom lens system. More particularly, but not exclusively, the present invention relates to a zoom lens system suitable as an optical system for an image pickup apparatus.
2. Description of the Related Art
Recently, the technology of solid-state image sensors, such as a CCD, used in image pickup apparatuses, such as digital still cameras and camcorders, has made remarkable progress. In addition, the image pickup apparatuses have been reduced in size. Therefore, a compact, thin, and lightweight photographic lens system having improved optical performance is needed.
A two-unit zoom lens has relatively improved optical performance, and its entire lens system is small. The two-unit zoom lens includes, from the object side to the image side, a first lens unit having negative refractive power and a second lens unit having positive refractive power, and changes the distance (air space) between the two lens units to perform zooming.
The lens system of this two-unit zoom lens is composed of a relatively small number of lenses. Therefore, this two-unit zoom lens is commonly used in small lens systems.
A small two-unit zoom lens whose first lens unit consists of a negative lens and a positive lens and whose second lens unit consists of a positive lens and a negative lens is discussed in Japanese Patent Laid-Open No. 6-273670, Japanese Patent Laid-Open No. 9-033810, and Japanese Patent Laid-Open No. 11-052235 (corresponding to U.S. Pat. No. 6,081,389).
A two-unit zoom lens whose second lens unit consists of a first lens subunit having positive refractive power and a second lens subunit having positive refractive power, the second lens subunit performing focusing, is discussed in Japanese Patent Laid-Open No. 2000-9997 (corresponding to U.S. Pat. No. 6,124,987).
A three-unit zoom lens suitable for a small image pickup apparatus having a high-resolution image sensor, the zoom lens including three lens units having negative, positive, and positive refractive power respectively, is discussed in Japanese Patent Laid-Open No. 2000-147381 (corresponding to U.S. Pat. No. 6,243,213) and Japanese Patent Laid-Open No. 2000-284177 (corresponding to U.S. Pat. No. 6,351,337).
A small three-unit zoom lens whose second lens unit consists of a positive lens and a negative lens is discussed in Japanese Patent Laid-Open No. 2000-9999 (corresponding to U.S. Pat. No. 6,172,818).
The two-unit zoom lenses and the three-unit zoom lenses discussed in the above documents can be improved in optical performance in order to be used for an image pickup apparatus using a high-resolution solid-state image sensor.